Referring to FIG. 1, a conventional oxide cathode comprises                a cathodo-emissive layer 3 basically made of alkaline earth oxides or a mixture of such oxides.        a substrate 1 onto which the cathodo-emissive layer 3 is deposited, which is generally made of a nickel alloy, containing one or several reducing agents as Mg, Al, Si, W, Cr and/or Zr. Nickel alloy of the substrate 1 is generally based on a mixture of nickel and tungsten or a mixture of nickel and molybdenum.        
The conventional oxide cathode shown on FIG. 1 comprises more precisely a cup-liked shape nickel alloy monolayer substrate 1, a tube-liked shape sleeve 2 made of an alloy containing at least Ni and Cr, onto which the metallic substrate 1 is welded. As two metallic parts are used to constitute the cathode, namely a metallic substrate 1 and a sleeve 2, this type of cathode is called a “two-piece cathode”. A cathodo-emissive layer 3 of double or triple carbonates, i.e. a mixture of (Ba, Sr) CO3 or (Ba, Sr, Ca) CO3 is deposited onto the substrate 1. Those carbonates, which are chemically stable under air exposure, are decomposed into double oxides BaO, SrO or triple oxides BaO, SrO, CaO by heating the cathode under vacuum during the activating sequence of a cathode-ray tube. Subsequently, metallic barium is created in the double or triple oxide cathodo-emissive layer 3 at the operating temperature of the cathode, which lies preferably in a range of 700° C. to 850° C., the presence of metallic barium being mainly responsible for the good emission properties of the double or triple oxide cathodo-emissive layer 3 of the cathode. For this point on, for convenience, references to triple oxide will be understood to include double oxide as well. The cathode is heated to its operating temperature by thermal radiation of a heater 4 inserted inside the sleeve 2; this heater 4 is generally made of a tungsten wire or an alloy of W and Re, generally covered by a layer of aluminum oxide.
As an alternate to the conventional cathode described in FIG. 1 having a nickel monolayer as a metallic substrate 1, a double layer of nickel alloys is also commonly used as a metallic substrate (see for example U.S. Pat. No. 3,919,751, G.T.E Sylvania Inc.). This double layer is commonly referred to as a bimetal. A cathode using a bimetal for a metallic substrate 1 is shown in FIG. 2. The double layer comprises a top layer 11 of nickel or an alloy of nickel containing 1 to 5% tungsten or alternatively 1 to 5% molybdenum (in weight percent) bonded to a bottom layer 12 of alloy of nickel and chromium (typically an alloy called “nichrome”, containing 20% Cr, the remainder being essentially Ni). The double layer substrate 1 comprising nickel top layer 11 and nickel-chromium bottom layer 12 can be formed into a cup which is welded onto a nickel-chromium sleeve 2, as described on FIG. 2. In this case, as a cup-shaped substrate 1 is welded to a sleeve 2, this cathode is also called a “two-piece cathode”, as the other conventional cathode described on FIG. 1. A usual cathodo-emissive layer 3 is made of triple carbonates, i.e. a mixture of (Ba, Sr, Ca) CO3, that is deposited onto the top layer 11. The deposited layer is heated to its operating temperature by thermal radiation of a heater 4 inserted into the sleeve 2 as in FIG. 1.
It is known, alternatively, to make a “one-piece cathode” using a bimetal, as described on FIG. 3. In this case, a bimetal strip which comprises a top layer 11 and a bottom layer 12 as previously described in reference to FIG. 2, is formed into a tube which is closed at one end, with the nickel-based top layer 11 appearing as the outside face of the tube. By selectively etching away the nickel-based top layer 11 and by protecting the closed end of the tube, it is possible to remove all the nickel-based top layer 11 except on the top-portion or closed end of the tube, thus leaving a double layer cap-shaped substrate 1 of a desired height on a nichrome sleeve 2 (See for example U.S. Pat. No. 4,849,066, R.C.A). This cathode, although obtained through a different process, is very similar to the previous cathode described in reference to FIG. 2. This cathode being constituted of only one part, is called a “one-piece cathode”.
In the oxide cathode, for both cases where the substrate 1 is made of a single layer or of a double layer, the creation of the metallic baryum is maintained through the cathode life by reduction of BaO into Ba caused by the chemical reaction of BaO with all elements contained in the nickel having a reducing power with respect to BaO. The chemical reduction occurs at the operating temperature of the cathode (typically 700° C.–850° C.) or at any step of the fabrication of the cathode ray tube where the cathode is heated, for instance the activation step designed to bring a cathode to its optimum emission capabilities. When the cathode is heated, the reducing elements contained in the substrate 1 thermally diffuse to the interface between the substrate 1 and the cathodo-emissive layer 3 where they react with BaO to liberate metallic Ba and form reaction compounds. Examples of chemical reactions between reducing elements and BaO are given below for Mg, Al, Si and W:Mg+BaO→MgO+Ba2 Al+4 BaO→BaAl2O4+3 BaSi+4 BaO→Ba2SiO4+2 BaW+6 BaO→Ba3WO6+3 Ba
Metallic barium, at the operating temperature of the cathode, constantly evaporates from the cathodo-emissive layer 3. To maintain good emission properties, this loss of barium must be compensated by the creation of metallic barium through chemical reactions as described above. The flux of reducing elements that react with BaO must not go below the minimum level necessary to create the amount of metallic barium needed for good emission properties to be sustained. The reducing elements come to the interface between the double-layer substrate 1 and the cathodo-emissive layer 3 by diffusion from the top layer 11 of the substrate. As the reducing elements contained in the bottom layer 12 (for example chrome, or Si if the “nichrome” of this bottom layer is Si doped) migrate from the bottom layer 12 into the top layer 11, as these reducing elements can also diffuse further to the interface between the top layer 11 and the cathodo-emissive layer 3 to play a positive role for cathode life, they act in fact as an additional reserve of reducing elements. On FIG. 9, the increase of silicon concentration in the top layer 11, measured by the ICP method (Inductively Coupled Plasma) as a function of operation time of such various cathodes is displayed. The initial concentration of silicon is the concentration on the metal of the top layer 11, as set at the elaboration of this metal. The enrichment of the top layer in silicon with time is attributed to the diffusion of the silicon from the bottom layer 12 to the top layer 11. In this series of bimetal samples, the average concentration of silicon in the nichrome bottom layer is 0.18%. The difference between this value of concentration in the bottom layer and the value in the nickel top layer is the driving force for the diffusion of Si from the bottom to the top layer.
It is of common usage to use either Mg or Zr as a fast activator acting at the beginning of cathode life combined with Si or Al as a long-term activator to extend cathode life when the fast activator is no longer acting. Two main factors are known to limit the flux of reducing elements to the interface between the top layer 11 and the cathodo-emissive layer 3. Firstly, as the reducing elements are consumed in the reaction with BaO, their concentration in the top layer 11 tends to decrease with life, and accordingly, their flux to this interface decreases. Complete exhaustion of the reducing elements can even occur if their initial concentration in the top layer 11 is low. Another factor limiting the flux of reducing elements is the build-up of the reaction compounds at this interface between the top layer 11 and the cathodo-emissive layer 3, forming thus a blocking layer for diffusing species (See for example: E. S. Rittner, Philips Res. Rep., T.8, p 184, 1953). First of all, this blocking layer starts to be created at the annealing steps performed on the substrate 1 prior to deposition of the cathodo-emissive layer 3. During this annealing step, the reducing elements are oxidized by minute amounts of oxygen resulting in the creation of MgO, SiO2 or Al2O3. The oxygen comes from the decomposition of the water vapor added in the atmosphere of the furnace used for annealing, usually composed of excess hydrogen. Then, this blocking layer is further built up during cathode operations. The interfacial compounds that mainly build-up during life are the W-based compounds and the Si-based compounds.
To overcome the exhaustion of the reducing agents, one could think of increasing the concentration of those latter in the top layer 11, but this has the disadvantage of increasing the rate of formation of the detrimental compounds that build-up the blocking layer. In addition to the disadvantage of limiting the flux of reducing elements, the interfacial compounds of this blocking layer tend to worsen the adhesion of the cathodo-emissive layer 3 on the top layer 11. This is in itself a sufficient reason to maintain the development of those compounds as low as possible. As a possible solution to avoid both the exhaustion of the reducing agents and the fast build-up of a blocking layer in the early stage of cathode life, it could be proposed to have a reasonably low concentration of reducing elements in the top layer 11, but a thickness of the substrate 1 high enough to provide a good reserve of reducing elements (See for example: H. E. Kern, Bell Laboratories Record, T. 38, no 12, p 451, December 1960). For instance, in a first approximation, one could consider about composition of the top layer 11 that a 400 μm thick nickel doped with 0.01% by weight of Si is equivalent, in terms of Si reserve, to 100 μm with 0.04% by weight of Si, the product of weight concentration by the thickness being the same. In fact, another important characteristic, i.e. the thickness E of the substrate 1, has to be taken into account for the design of the cathodes for cathode ray tubes, such as display tubes for computers or television. The electron-beam turn-on time is directly linked to the time needed for the cathode to reach its operating temperature. This time increases with the cathode weight so it is of importance to have the substrate 1 with the lowest possible weight. The lowest thickness commonly used for cathode metallic substrate 1 is about 70–100 μm, but such a low thickness forbids the use of low concentrations of reducing elements in this substrate, because firstly, at least 1% of W and/or Mo in weight in the nickel used for the substrate is necessary to maintain a good mechanical strength of the substrate and secondly, the concentration of the active reducing elements like Mg or Si cannot be set at low levels around 0.01% in weight because the reserve of reducing elements would be too low. If the thickness of the substrate is increased in a range of 150 to 200 μm, far lower concentration of W and/or Mo can be used for the substrate based on nickel, but the turn-on-time is degraded in comparison with a substrate thickness of 70 μm. Thus, it seems that there is no way to combine both the low concentration of reducing elements leading to a slow rate of formation of detrimental compounds and of a blocking layer, and the low substrate thickness leading to the low turn-on time. This could be done only at the expense of cathode lifetime, a case that is not acceptable. The present invention offers the possibility of both a minimized turn-on-time and a long lifetime.
Another important aspect of the optimization of the substrate chemistry is the fact that some of the reducing elements have a relatively high vapor pressure that leads to significant evaporation into the vacuum of the cathode ray-tube, when the cathode is heated. Comprised in this family of high vapor pressure elements is Mg. The Mg metallic vapor tends to condense on the different parts of the electron gun of the cathode ray-tube. For all types of cup-liked shape metallic substrate, one can define top face 111 as the surface on which the cathodo-emissive layer 3 is deposited and bottom face 122 as the surface of the substrate which is opposite to top face 111, as shown on FIG. 3. Bottom face 122 faces the heater 4 and several electrical metallic connectors of the electron gun which are not represented. The deposition of the metallic vapors of reducing elements coming from the cathode onto these electrical connectors create electrical leakages or shortcuts between electrodes of the gun detrimental to the good operation of the electron gun in the cathode-ray tube.